Secure environments for communicating information is an ongoing concern in the field of communications. Historically, government agencies have used many different types of data encryption to obviate interception of communicated information between parties. Commercially, several techniques have been deployed to reduce or eliminate communications from being intercepted. For example, some conventional wireless telephone systems deploy spread spectrum as a form of wireless communication in which the frequency of the transmitted signal is deliberately varied resulting in greater bandwidth than the signal would have if its frequency were not varied. For example, a conventional wireless signal has a frequency, usually specified in megahertz(MHz) or gigahertz (GHz), that does not change with time (except for small, rapid fluctuations that occur as a result of modulation). The signal will stay at approximately 103.1 MHz and may vary up to 105.1 MHz or down to 99.1 MHz. Some conventional wireless telephones maintain the signal constant so the bandwidth can be kept within certain limits and the signal can be easily located by someone who wants to retrieve the information.
Given that a constant frequency signal is relatively easy to intercept, it is therefore not well suited to applications in which information must be kept confidential between the source (transmitting party) and destination (receiving party).
To minimize troubles that can arise from the above mentioned vulnerabilities of conventional communications circuits, the frequency of the transmitted signal can be deliberately varied over a comparatively large segment of the electromagnetic radiation spectrum. This variation is done according to a specific, but complicated mathematical function. In order to intercept the signal, a receiver must be tuned to frequencies that vary precisely according to this function. The receiver must “know” the frequency-versus-time function employed by the transmitter, and must also “know” the starting-time point at which the function begins. If someone wants to jam or intercept a spread-spectrum signal, that person must have a transmitter that “knows” the function and its starting-time point. The spread-spectrum function must be kept out of the hands of unauthorized people or entities to protect security of the associated communication.
Most spread-spectrum signals use a digital scheme called frequency hopping. The transmitter frequency changes abruptly, many times each second. Between “hops,” the transmitter frequency is stable. The length of time that the transmitter remains on a given frequency between “hops” is known as the dwell time. A few spread-spectrum circuits employ continuous frequency variation, which is an analog scheme.